The use of electric motors together with a gasoline engine (hybrid vehicle) as well as a conversion to an electric motor (electric vehicle) is progressing as a countermeasure to global warming in order to reduce the amount of carbon dioxide generated from an automobile. When using an electric motor as a source to generate motive power in a vehicle, an in-wheel motor system is known in which an electric motor and braking device are assembled in a wheel space (see Japanese Unexamined Patent Application Publication No. 2004-152416A and Japanese Unexamined Patent Application Publication No. 2004-115014A). Further, because the structure of the in-wheel motor system arranges the electric motor in the interior of the wheel, heat generated by the electric motor easily builds up in the interior of the wheel. Therefore, a structure that allows the heat generated by the electric motor to easily dissipate is desired. Techniques, such as those described in Japanese Unexamined Patent Application Publication No. H05-104969A and Japanese Unexamined Patent Application Publication No. 2006-246678A, are known as techniques for improving heat dissipation in an in-wheel motor system. The in-wheel motor system contributes to the compactness of a vehicle because a significant area can be made available within the vehicle while eliminating power train components such as a drive shaft, differential gear, and the like. Further, the in-wheel motor system is garnering attention as an electric vehicle (EV) system of the future for the ease in drive control of the steering wheels and in manifesting a drive performance not available in conventional vehicles. For example, a vehicle that is equipped with in-wheel motors in either four wheels or in the left and right wheels can be expected to utilize the high responsivity, precision of the applied torque, and the reversible rotary characteristic of the electric motors to enable a high degree of vehicle attitude control and to exhibit high vehicle maneuverability.